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Clostridium difficile toxin B (TcdB) is a critical virulence factor that causes diseases associated with C. difficile infection. Here we carried out CRISPR–Cas9-mediated genome-wide screens and identified the members of the Wnt receptor frizzled family (FZDs) as TcdB receptors. TcdB binds to the conserved Wnt-binding site known as the cysteine-rich domain (CRD), with the highest affinity towards FZD1, 2 and 7. TcdB competes with Wnt for binding to FZDs, and its binding blocks Wnt signalling. FZD1/2/7 triple-knockout cells are highly resistant to TcdB, and recombinant FZD2-CRD prevented TcdB binding to the colonic epithelium. Colonic organoids cultured from FZD7-knockout mice, combined with knockdown of FZD1 and 2, showed increased resistance to TcdB. The colonic epithelium in FZD7-knockout mice was less susceptible to TcdB-induced tissue damage in vivo . These findings establish FZDs as physiologically relevant receptors for TcdB in the colonic epithelium. Here, a genome-wide CRISPR–Cas9 screen is used to identify the Wnt receptors frizzled as physiologically relevant Clostridium difficile toxin B receptors, providing new therapeutic targets for treating C. difficile infections. Frizzled drug targets in C. difficile Clostridium difficile infection is the most common cause for antibiotic-associated diarrhea and the leading cause of gastroenteritis-associated death in developed countries. Two homologous toxins, toxin A (TcdA) and toxin B (TcdB), enter host cells through receptor-mediated endocytosis and are the causal agents of C. difficile pathology. How TcdB targets the colonic epithelium has been unclear. In this study, Min Dong et al . use a genome-wide CRISPR screen to identify members of the Wnt receptor frizzled family (FZDs) as physiologically relevant TcdB receptors in the murine colonic epithelium and in colonic organoids, with highest affinity toward FZD1, 2 and 7. In so doing, this work provides new therapeutic targets for treating C. difficile infections.

β-Catenin signaling controls the development and maintenance of the blood–brain barrier (BBB) and the blood–retina barrier (BRB), but the division of labor and degree of redundancy between the two principal ligand–receptor systems—the Norrin and Wnt7a/Wnt7b systems—are incompletely defined. Here, we present a loss-of-function genetic analysis of postnatal BBB and BRB maintenance in mice that shows striking threshold and partial redundancy effects. In particular, the combined loss of Wnt7a and Norrin or Wnt7a and Frizzled4 (Fz4) leads to anatomically localized BBB defects that are far more severe than observed with loss of Wnt7a, Norrin, or Fz4 alone. In the cerebellum, selective loss of Wnt7a in glia combined with ubiquitous loss of Norrin recapitulates the phenotype observed with ubiquitous loss of both Wnt7a and Norrin, implying that glia are the source of Wnt7a in the cerebellum. Tspan12, a coactivator of Norrin signaling in the retina, is also active in BBB maintenance but is less potent than Norrin, consistent with a model in which Tspan12 enhances the amplitude of the Norrin signal in vascular endothelial cells. Finally, in the context of a partially impaired Norrin system, the retina reveals a small contribution to BRB development from the Wnt7a/Wnt7b system. Taken together, these experiments define the extent of CNS region-specific cooperation for several components of the Norrin and Wnt7a/Wnt7b systems, and they reveal substantial regional heterogeneity in the extent to which partially redundant ligands, receptors, and coactivators maintain the BBB and BRB.

A human neurodevelopmental model fills the current knowledge gap in the cellular biology of Williams syndrome and could lead to further insights into the molecular mechanism underlying the disorder and the human social brain. An iPSC model for Williams syndrome Individuals with the neurodevelopmental disorder Williams syndrome (WS) lack a region of about 25 genes on chromosome 7. The condition is characterized by hypersociability and a range of cognitive and behavioural impairments, but how specific genes contribute to the neuroanatomical and functional alterations is not known. Alysson Muotri and colleagues have used cellular reprogramming technologies to generate induced pluripotent stem cells (iPSCs) from individuals with WS and controls. iPSC-derived neural progenitor cells from individuals with WS had increased apoptosis owing to haploinsufficiency of the gene FZD9. In addition, iPSC-derived WS cortical neurons displayed altered activity and morphological changes, some of which matched those seen in postmortem brains of individuals with WS. This human iPSC model may provide insights into the molecular and cellular mechanisms underlying the various features of the disorder. Williams syndrome is a genetic neurodevelopmental disorder characterized by an uncommon hypersociability and a mosaic of retained and compromised linguistic and cognitive abilities. Nearly all clinically diagnosed individuals with Williams syndrome lack precisely the same set of genes, with breakpoints in chromosome band 7q11.23 (refs 1 , 2 , 3 , 4 , 5 ). The contribution of specific genes to the neuroanatomical and functional alterations, leading to behavioural pathologies in humans, remains largely unexplored. Here we investigate neural progenitor cells and cortical neurons derived from Williams syndrome and typically developing induced pluripotent stem cells. Neural progenitor cells in Williams syndrome have an increased doubling time and apoptosis compared with typically developing neural progenitor cells. Using an individual with atypical Williams syndrome 6 , 7 , we narrowed this cellular phenotype to a single gene candidate, frizzled 9 ( FZD9 ). At the neuronal stage, layer V/VI cortical neurons derived from Williams syndrome were characterized by longer total dendrites, increased numbers of spines and synapses, aberrant calcium oscillation and altered network connectivity. Morphometric alterations observed in neurons from Williams syndrome were validated after Golgi staining of post-mortem layer V/VI cortical neurons. This model of human induced pluripotent stem cells 8 fills the current knowledge gap in the cellular biology of Williams syndrome and could lead to further insights into the molecular mechanism underlying the disorder and the human social brain.

Changing the morphology of a simple epithelial tube to form a highly ramified branching network requires changes in cell behavior that lead to tissue-wide changes in organ shape. How epithelial cells in branched organs modulate their shape and behavior to promote bending and sculpting of the epithelial sheet is not well understood, and the mechanisms underlying this process remain obscure. We show that the Wnt receptor Frizzled 2 (Fzd2) is required for domain branch formation during the initial establishment of the respiratory tree. Live imaging and transcriptome analysis of lung-branching morphogenesis demonstrate that Fzd2 promotes changes in epithelial cell length and shape. These changes in cell morphology deform the developing epithelial tube to generate and maintain new domain branches. Fzd2 controls branch formation and the shape of the epithelial tube by regulating Rho signaling and by the localization of phospho-myosin light chain 2, in turn controlling the changes in the shape of epithelial cells during morphogenesis. This study demonstrates the importance of Wnt/Fzd2 signaling in promoting and maintaining changes in epithelial cell shape that affect development of a branching network.

Targeted mutation of the Frizzled3 (Fz3) gene in mice has been shown to disrupt the growth and guidance of a subset of peripheral and central axons. Here we used conditional deletion of Fz3 to explore the forebrain territories in which Fz3 action is required for the development of the anterior commissure and the corticothalamic, corticospinal, and thalamocortical tracts. Experiments with region-specific deletion of Fz3 using a variety of Cre lines show that proper routing of corticothalamic and thalamocortical axons in the internal capsule requires Fz3 expression in the ventral telencephalon. The pattern of defects among forebrain axon tracts that are induced by conditional deletion of Fz3 conforms closely to the pattern previously observed with analogous conditional deletion of Celsr3 , implying a close mechanistic link between Fz3 and Celsr3 in axon guidance. We further found that several central nervous system axon tracts require Fz3 function as early as embryonic day 11.5, and that Fz3 is required for pathfinding by dopaminergic and serotonergic axons in the brain and by a subset of optic tract axons. In addition, conditional deletion of Fz3 in all tissues caudal to the neck eliminates the spinothalamic tract and the transmission of somatosensory information from the spinal cord to the brain, as determined by neuroanatomic tracing and behavioral testing.

Nails protect the soft tissue of the tips of digits. The molecular mechanism of nail (and claw) development is largely unknown, but we have recently identified a Wnt receptor gene, Frizzled6 (Fzd6), that is mutated in a human autosomal-recessive nail dysplasia. To investigate the action of Fzd6 in claw development at the molecular level, we compared gene expression profiles of digit tips of wild-type and Fzd6−/- mice, and showed that Fzd6 regulates the transcription of a striking number of epidermal differentiation–related genes. Sixty-three genes encoding keratins (Krts), keratin-associated proteins, and transglutaminases (Tgms) and their substrates were significantly downregulated in the knockout mice. Among them, four hard Krts, Krt86, Krt81, Krt34, and Krt31; two epithelial Krts, Krt6a and Krt6b; and Tgm 1 were already known to be involved in nail abnormalities when dysregulated. Immunohistochemical studies revealed decreased expression of Krt86, Krt6b, and involucrin in the epidermal portion of the claw field in the knockout embryos. We further showed that Dkk4, a Wnt antagonist, was significantly downregulated in Fzd6−/- mice along with Wnt, Bmp, and Hh family genes; and Dkk4 transgenic mice showed a subtly but appreciably modified claw phenotype. Thus, Fzd6-mediated Wnt signaling likely regulates the overall differentiation process of nail/claw formation.

The mammalian hair follicle unit consists of a central follicle and a series of associated structures: sebaceous glands, arrector pili muscles, Merkel cells, and sensory nerve endings. The architecture of this multicellular structure is highly polarized with respect to the body axes. Previous work has implicated Frizzled6 (Fz6)-mediated planar cell polarity (PCP) signaling in the initial specification of hair follicle orientation. Here we investigate the origin of polarity information among structures within the hair follicle unit. Merkel cell clusters appear to have direct access to Fz6-based polarity information, and they lose polarity in the absence of Fz6. By contrast, the other follicle-associated structures likely derive some or all of their polarity cues from hair follicles, and as a result, their orientations closely match that of their associated follicle. These experiments reveal the interplay between global and local sources of polarity information for coordinating the spatial arrangement of diverse multicellular structures. They also highlight the utility of mammalian skin as a system for quantitative analyses of biological polarity.

Progression of colorectal cancer (CRC) involves spatial and temporal occurrences of epithelial–mesenchymal transition (EMT), whereby tumour cells acquire a more invasive and metastatic phenotype. Subsequently, the disseminated mesenchymal tumour cells must undergo a reverse transition (mesenchymal–epithelial transition, MET) at the site of metastases, as most metastases recapitulate the pathology of their corresponding primary tumours. Importantly, initiation of tumour growth at the secondary site is the rate-limiting step in metastasis. However, investigation of this dynamic reversible EMT and MET that underpins CRC morphogenesis has been hindered by a lack of suitable in vitro models. To this end, we have established a unique in vitro model of CRC morphogenesis, which we term LIM1863- Mph ( m or ph ogenetic). LIM1863- Mph cells spontaneously undergo cyclic transitions between two-dimensional monolayer (migratory, mesenchymal) and three-dimensional sphere (carcinoid, epithelial) states. Using RNAi, we demonstrate that FZD7 is necessary for MET of the monolayer cells as loss of FZD7 results in the persistence of a mesenchymal state (increased SNAI2/decreased E-cadherin). Moreover, FZD7 is also required for migration of the LIM1863- Mph monolayer cells. During development, FZD7 orchestrates either migratory or epithelialization events depending on the context. Our findings strongly implicate similar functional diversity for FZD7 during CRC morphogenesis.

R-spondin proteins strongly potentiate Wnt signalling and function as stem-cell growth factors. Despite the biological and therapeutic significance, the molecular mechanism of R-spondin action remains unclear. Here we show that the cell-surface transmembrane E3 ubiquitin ligase zinc and ring finger 3 (ZNRF3) and its homologue ring finger 43 (RNF43) are negative feedback regulators of Wnt signalling. ZNRF3 is associated with the Wnt receptor complex, and inhibits Wnt signalling by promoting the turnover of frizzled and LRP6. Inhibition of ZNRF3 enhances Wnt/β-catenin signalling and disrupts Wnt/planar cell polarity signalling in vivo . Notably, R-spondin mimics ZNRF3 inhibition by increasing the membrane level of Wnt receptors. Mechanistically, R-spondin interacts with the extracellular domain of ZNRF3 and induces the association between ZNRF3 and LGR4, which results in membrane clearance of ZNRF3. These data suggest that R-spondin enhances Wnt signalling by inhibiting ZNRF3. Our study provides new mechanistic insights into the regulation of Wnt receptor turnover, and reveals ZNRF3 as a tractable target for therapeutic exploration. ZNRF3 and RNF43 are identified as negative feedback regulators of Wnt signalling; the stem-cell growth factor R-spondin is shown to potentiate Wnt signalling by inhibiting ZNRF3. ZNRF3 protein inhibits Wnt signalling The R-spondin proteins are secreted molecules that function as stem-cell growth factors and potentiate Wnt signalling by binding LGR4 family receptors, but their precise mechanism of action remains unclear. Here, the transmembrane E3 ubiquitin ligase ZNRF3 is identified as an inhibitor of Wnt signalling that acts by promoting the turnover of Wnt receptors. R-spondin potentiates Wnt signalling by inhibiting ZNRF3 in a mechanism dependent on LGR4, resulting in the accumulation of Wnt receptors. Given the importance of Wnt signalling in cancer, ZNRF3 may be a target for therapeutic intervention.

In vivo and in vitro studies show that the stem-cell E3 ubiquitin ligases RNF43 and ZNRF3 act as tumour suppressors in colorectal cancer models, and are involved in the negative regulation of the cancer-associated Wnt signalling pathway through limiting the cell-surface expression of Wnt receptors. RNF43 protein in Wnt-linked colon cancer Wnt signalling is critical for the function of intestinal stem cells; it also drives colorectal tumorigenesis. Bon-Kyoung Koo et al . find that two targets of Wnt signalling, the E3 ligases RNF43 and ZNFR3, are also important negative-feedback regulators of Wnt signalling. They act by limiting the cell-surface expression of Wnt receptors. Deletion of both genes in the mouse intestine leads to expansion of LGR5 + intestinal stem cells and the development of adenomas. Furthermore, in human colon cancer cells, the expression of RNF43 reduces Wnt signalling. Mutated RNF43 has been found in human colorectal cancers, indicating that Wnt-pathway inhibitors that act at the level of Wnt secretion or Wnt-receptor activation may have therapeutic potential. LGR5 + stem cells reside at crypt bottoms, intermingled with Paneth cells that provide Wnt, Notch and epidermal growth factor signals 1 . Here we find that the related RNF43 and ZNRF3 transmembrane E3 ubiquitin ligases are uniquely expressed in LGR5 + stem cells. Simultaneous deletion of the two genes encoding these proteins in the intestinal epithelium of mice induces rapidly growing adenomas containing high numbers of Paneth and LGR5 + stem cells. In vitro , growth of organoids derived from these adenomas is arrested when Wnt secretion is inhibited, indicating a dependence of the adenoma stem cells on Wnt produced by adenoma Paneth cells. In the HEK293T human cancer cell line, expression of RNF43 blocks Wnt responses and targets surface-expressed frizzled receptors to lysosomes. In the RNF43 -mutant colorectal cancer cell line HCT116, reconstitution of RNF43 expression removes its response to exogenous Wnt. We conclude that RNF43 and ZNRF3 reduce Wnt signals by selectively ubiquitinating frizzled receptors, thereby targeting these Wnt receptors for degradation.